1. Field of the Invention
Polymers having high-temperature characteristics are required to improve the performance and to reduce the weight of industrial materials in electronic devices, aeronautical equipment and in some machinery. The polyimides and polyacrylates are polymers known to have the required mechanical strength, dimensional stability, low coefficient of thermal expansion, and electrical insulation properties in addition to high-temperature resistance.
The preparation of high performance polymers, however, requires cure temperatures in excess of 200° C. This leads to high tooling costs, high processing costs, and processing induced thermal stresses that can compromise component durability. The process of this invention allows the curing of high performance polyimides and polyacrylates at or near room temperature. This invention enables the cure of high performance polyimides and polyacrylates at or near room temperature by using ultraviolet light or some other radiation sources, such as electron beams rather than heat to provide the cure energy. Specifically, this invention relates to the Diels-Alder cyclopolymerization of photochemically generated dienes with dienophiles, such as bismaleimides and mixtures thereof with a maleimide end-cap and trismaleimides. Irradiation of an aromatic diketone produces two distinct hydroxy o-quinodimethane (photoenol) intermediates. The intermediates are trapped via a Diels-Alder cycloaddition with appropriate dienophiles, e.g., bismaleimides and/or trismaleimides to give the corresponding polyimides in quantitative yields. When maleimides such as bismaleimide and/or trismaleimide are used as the dienophile, the resulting polyimides of this invention have glass transition temperatures, (Tg), as high as 300° C.
2. Description of the Prior Art
The preparation of high-performance polymers such as polyimides or polyesters are typically prepared by condensation reactions. In the case of polyimides, the reaction involves diamines and dianhydrides or dianhydride derivatives e.g., the diester of tetracarboxylic acids. This process suffers from several problems in that aromatic diamines are toxic, mutagenic, or carcinogenic. Safe handling and disposal of these materials requires the implementation of costly engineering controls. Further, processing of condensation reaction systems also can be a problem, since this chemistry leads to low molecular weight by-products, e.g., water and alcohols. Evolution of these by-products and high processing temperatures lead to voids and defects in the polymer and the composites prepared with these polymers.
It is known that some of these processing problems can be overcome, however, by combining addition chemistry with condensation chemistry, as is the case for PMR-15 polyimides. With this approach, low molecular weight oligomers (short chain polymers) are prepared by the condensation of diamines with dianhydrides or its derivatives and a suitable endcapping group. The endcaps undergo a cross-linking reaction at high temperatures (typically in excess of 300° C.) to provide a polymer network having good solvent resistance and high temperature performance. Prior to cross-linking, however, the imide oligomers are somewhat fluid, and volatile condensation by-products can be removed from the polymer. While this approach overcomes some of the processing difficulties, it requires higher processing temperatures and monomer toxicity is still a concern.
It is known also in the prior art that the Diels-Alder polymerization reaction has been used to prepare high performance polymers such as the polyimides and polyesters. Typical Diels-Alder reactions used to obtain polyimides have involved the reaction of bismaleimides with a suitable bisdiene such as a bisfuran. Other Diels-Alder reactions use a bisdiene precursor, such as bis(benzocyclobutane), to form the bisdiene upon heating to temperatures of 250° C. or higher. Using these Diels-Alder cyclopolymerization reactions overcome the health and safety problems associated with other methods of preparing polyimides, since these reactions do not involve the use of aromatic amines as one of the reactants. However, these methods still require high cure and processing temperatures; see, for example, U.S. Pat. Nos. 5,338,827; 5,322,924; 4,739,030 and the Annual Reviews in Materials Science, 1998, 28, 599–630 by M. A. Meador.